BUG: account for None in bounds in SLSQP

In line 323 of slsqp.py, float conversion leads to nan if Nones were initially
present in bounds. Checking for isfinite (instead of isinf) filters both inf
and nan.

Related to ticket #1851.
TST: test SLSQP with None in bounds
DOC: mention that COBYLA does not support bounds

Also correctly test string equality using == instead of `is`.
Merge pull request #449 from dlax/ticket-1851

BUG: optimize: account for None in bounds in SLSQP